The present invention relates an electric motor having power supply brushes and a commutator of an armature.
In conventional direct current motors having power supply brushes, multiple (for instance, four) brushes are used to reduce load current for each brush when high output is required. It is most desired that two same-polarity brushes contact two segments of a commutator at the same time.
It is however difficult in practice to cause the two same-polarity brushes to contact the corresponding segments at the same time. As a result, the timings of contact of the brushes differ. The timing of power supply to the armature is imbalanced when current flows in one of armature windings. This imbalance will cause fluctuation in the motor rotation, vibration, unusual noise and the like.
These fluctuation in the motor rotation, vibration and unusual noise arising secondarily will lessen response characteristics in operation, cause discomfort and the like in the case that the motor is applied, for instance, to home appliances and automotive devices such as an electric power steering device.
In the conventional motor, therefore, the windings are shorted by shorting wires between the same-phase segments to provide strapping connection. This connection reduces the fluctuation in the motor rotation, vibration and unusual noise.
The conventional strapping connection requires a separate special winding process. The winding work increases, because each shorting wire is wound around a slotted core of an armature. The armature and hence the electric motor thus result in high cost. Further, the shorting wire would cause undesirable influence to the armature in respect of inductance and the like, because it is wound a rather long distance.
Further, in the armature with the strapping connection, the conventional brushes which contact the segments of the armature has a narrow segment-contacting surface. The brush has only a width sufficient to bridge at most two segments at the same time. As a result, the brushes of the same polarity cannot contact the same-phase segments at the same time, when the interval between the same-polarity brushes varies. This results in variations in the contact timing.
That is, when one brush contacting a segment leaves from one segment and contacts the next segment, it may happen that the other brush cannot contact another segment which follows a segment which is in the same phase as the one segment contacted by the one brush. In this instance, when the one segment leaves the one segment and contacts the next segment, the voltage applied to from the brush to the next segment cannot be absorbed causing sparks and noises. This is an impediment to prolong the life of the electric motor and to reduce noises.